When you enter the Time Machine browser, these snapshots will appear in the timeline along with regular backups, but they have a different color. According to Apple, “Gray tick marks represent local snapshots and pink tick marks represent backups stored on your external backup disk or Time Capsule. Note: Pink tick marks will be dimmed if your portable computer is not connected to your external backup disk or Time Capsule.”
Note pink and white tickmarks on the right. (Credit: Apple, Inc.)
The article also shows how you can use the “About This Mac” window (under the Apple menu) to show how much disk space is being used by the snapshots.” Lion will surrender some of the space used by these backups if required.